1. Field of the Invention
The present invention relates to an error check or error correction code coding device and, more particularly, to suppression of a low-frequency component of a code string including an error check or error correction code.
2. Description of the Related Background Art
In general, in a system for converting an information signal such as an image signal into a digital signal, and transmitting the digital signal onto a transmission path of, e.g., a recording medium, information data is converted to a transmission code suitable for the transmission path, and the transmission code is transmitted.
In this specification, a magnetic recording apparatus such as a digital VTR will be exemplified as a typical transmission apparatus.
In a magnetic recording apparatus of this type, it is difficult to record/reproduce low-frequency or DC components due to transmission characteristics of a magnetic recording system. For this reason, in general, digital data to be recorded is converted to a recording code including less low-frequency components, and recording is then performed.
As a conversion coding system for suppressing low-frequency components, a conversion coding system having redundancy such as a system for converting 8-bit data into 9-bit data (8-9 conversion) is normally employed. With this system, however, redundancy is undesirably increased. Along with an increase in data volume and an increase in recording density, a demand has arisen for a coding system which is free from an increase in redundancy in consideration of a requirement for recording/reproduction with a smaller number of codes.
As a system free from an increase in redundancy, for example, an n-n mapping coding system for converting n-bit data to n-bit data is proposed. The n-n mapping coding suppresses low-frequency components of a code string to be recorded by utilizing a statistical nature of an input code string, e.g., for image information, the which correlation between adjacent codes.
As an example of this system, by utilizing the fact that input signals are converted to differential codes and the differential codes are concentrated near zero of positive/negative quantization level, i.e., define a Laplace distribution, a code having a small CDS (Code word Digital Sum) is assigned to a differential code having a high frequency of appearance, thereby decreasing a DSV (Digital Sum Value) of a mapping-coded code string after conversion. In this manner, low-frequency components of a code string to be coded are suppressed. For example, a 4-4 mapping coding system for converting a 4-bit differential code into a 4-bit code is known.
The mapping coding can suppress low-frequency components of a coded code string for an information code such as image information having a correlation between adjacent codes, as described above. However, the mapping coding cannot suppress low-frequency components of codes having no such correlation.
For example, when an error check or correction code for checking or correcting a code error or additional information having no correlation is added or substituted in a code string to be recorded, sufficient suppression of low-frequency components cannot be obtained for the code string. As the result, a code error rate upon decoding may be undesirably increased.
This respect will be further described below with reference to FIG. 1. FIG. 1 is a view showing a format of a typical data frame as a format of a code string to be recorded. In FIG. 1, an information code string subjected to mapping coding described above is stored in a portion shown as "information data", and a check digit of an error check or correction code, e.g., a Hamming code, or Reed-Solomon code is stored in a portion shown as "parity". Furthermore, an additional information code such as a synchronizing code or an ID code is stored in a portion shown as "synchronizing signal".
When the data frame is constituted, as shown in FIG. 1, a portion in which check digits of an error check or correction code successively appear has no correlation, and cannot be subjected to mapping coding. Thus, the same codes tend to continuously appear. Therefore, low-frequency components tend to be generated in this portion, and cannot be sufficiently suppressed in the entire code string to be recorded.
FIG. 2 shows a data matrix in which an inner code (row check code) is formed in the data frame shown in FIG. 1, and a plurality of data frames are vertically arranged to form an outer code (column check code), so that a product code is formed as a whole. In this format, in particular, since main information codes and parities are two-dimensionally arranged, this code format is suitable for an apparatus for recording a code string obtained by coding two-dimensional information such as image data. The data matrix is sequentially recorded in units of data frames.
When the data matrix shown in FIG. 2 is formed, suppression of low-frequency components by mapping coding cannot be expected since the inner and outer codes have no correlation therebetween, and the same codes tend to continuously appear. In particular, in a data frame constituted by parities of codes and inner codes, the parities continuously appear for a long period of time, and low-frequency component suppression in code strings near this code frame is considerably impaired.
As a method of solving such a problem, the assignee of the present invention proposed a technique for scattering additional information codes such as error check or correction codes in a code string to be recorded (see U.S. Pat. No. 4,779,276).
In this technique, since codes which cause generation of low-frequency components are scattered in a code string, the code error rate in decoding can be greatly reduced. In this technique, low-frequency components of error check or correction codes themselves are left unchanged.